Spatial light modulator device for the modulation of a wave field with complex information

ABSTRACT

A three-dimensional light modulator, of which the pixels are combined to form modulation elements. Each modulation element can be coded with a preset discrete value such that three-dimensionally arranged object points can be holographically reconstructed. The light modulator is characterized in that assigned to the pixels of the modulator are beam splitters or beam combiners which, for each modulation element, combine the light wave parts modulated by the pixels by means of refraction or diffraction on the output side to form a common light beam which exits the modulation element in a set propagation direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/356,084, filed on Mar. 18, 2019, which is a continuation of U.S.application Ser. No. 13/380,178, filed on Dec. 22, 2011, which claimsthe priority of PCT/EP2010/058626, filed on Jun. 18, 2010, which claimspriority to European Application No. 09163528.4, filed Jun. 23, 2009 andGerman Application No. 10 2009 044910.8, filed Sep. 23, 2009, the entirecontents of which are hereby incorporated in total by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a spatial light modulator device forthe modulation of a light wave field with video hologram information, inparticular with discrete complex object light point values ofthree-dimensional scenes whose object light points are to bereconstructed holographically. The invention is preferably applicable inthe context of a holographic reconstruction system that comprises aposition controller, an eye finder and an optical wave tracking meanswhich tracks the optical axis of the propagating modulated light wavefield to the actual eye position when an observer who watches theholographic information changes their position. Such a holographicreconstruction system has been disclosed for example in WO 2006/119760A2.

The invention can be used independent of the way in which theholographic information is provided, and it can also be implemented in asystem which allows multiple observers to watch holographicallyreconstructed video scenes simultaneously.

As is generally known, in order to reconstruct three-dimensional scenesby means of video holography, a light wave generator generates adirected light wave field which emits light waves which are capable ofgenerating interference towards the spatial light modulator device. Toallow easy addressability of the light modulator device it preferablycomprises a regular structure of modulator elements each of which beingencoded by a modulator controller with a discrete complex hologram valuein accordance with the spatial arrangement of object light points in avideo scene to be reconstructed.

In the present document, ‘encoding’ shall be understood to be thediscrete adjustment of the optical transmittance of modulator cells ofthe light modulator device. As a result of this encoding, the modulatorcells modulate incident light wave portions of the light wave fieldwhich is capable of generating interference such that multiple emittedlight wave portions reconstruct by way of constructive or destructiveinterference the plurality of object light points of a scene that isdescribed by a video signal in the space on the optical path downstreamof the spatial light modulator means, seen in the direction of lightpropagation.

In the context of the present invention, discrete complex hologramvalues carry holographic information for discrete encoding of amodulation array with a video hologram. The modulator controller writesto each modulator element components of the holographic code, such as areal part and an imaginary part in terms of complex numbers, in order toaffect amplitude and/or phase of the light transfer function of eachmodulator element.

Conventional light modulator devices usually serve either asamplitude-only modulators or as phase-only modulators, thus onlyinfluencing the light waves with a single real value. This means thatthese modulators locally change either the amplitude only or the phaseinformation only through their cell encoding.

In video holography, light modulator devices must be capable of workingin real-time, and they must be able to produce full-colourreconstructions in a large reconstruction space.

Each light modulator device comprises at least one modulation array withregularly arranged modulator elements, where each modulator elementcomprises a number of modulator cells. A modulation array is typicallyrealised by a spatial light modulator (SLM). Such a spatial lightmodulator has individual modulator cells, commonly also known as pixels.

According to the principle of hologram reconstruction, the modulatorcontroller simultaneously computes discrete complex hologram values forall modulator elements which are involved in the holographicreconstruction of an object light point based on the correspondingdiscrete complex value of the object light point of the scene.Corresponding code value components are generated for each discretecomplex hologram value prior to the encoding process. The code valuecomponents for each modulator element are computed and mutually adjustedsuch that all modulator cells of each modulator element interact suchthat the complex local light modulation which is expected from themodulator element is actually achieved. The complex object light pointvalues are computed by the modulator controller prior to the encodingfor example based on a video signal with depth information.

Document U.S. Pat. No. 5,416,618, for example, discloses a lightmodulator device which comprises a combination of multiple stackedspatial light modulator arrays. For example, one light modulator arraywith amplitude light modulator cells and one with phase light modulatorcells, or two arrays with light modulator cells of the same kind, arestacked in the direction of light propagation. This stacking generatesthe modulator elements which comprise multiple single cells and whichmodulate a light wave field with complex hologram values. It is adisadvantage here, however, that when the light modulator arrays areadjoined, considerable adjustment efforts must be made in order torealise exact congruence of the cell structures.

This disadvantage does not occur though where a complex hologram valueis realised by a group of multiple modulator cells of one modulationarray, in particular when the multiple modulator cells are arranged sideby side in respect of the direction of light propagation.

The light modulator device according to the present applicationtherefore comprises at least one modulation array with regularlyarranged modulator cells which can be encoded discretely but which arecombined to form joint modulation elements as regards their opticaleffect and their electrical addressing. The modulation elements of thelight modulator device are arranged in an array and modulate in aspatially structured way the light waves which are capable of generatinginterference of the propagating light wave field. This means that eachmodulation element only changes a light wave portion of the propagatinglight wave field which actually hits the modulation array in accordancewith the actual hologram value for the modulation element. For this, themodulator controller provides for each individual modulator cell aseparate value component of the complex object scanning value which isassigned to the modulation element.

Each modulation element thus comprises a combination of modulator cells,where the modulator cells can be realised in the form ofphase-modulating light modulator cells or amplitude-modulating lightmodulator cells. This means that, depending on the design and localarrangement of the modulator cells, each modulation element can modulatea light wave portion of the impingent propagating light wave field asregards its wave phase with one modulator cell and its wave amplitudewith the other modulator cell, or as regards its wave phase or waveamplitude only with all modulator cells.

The general principle of the spatial light modulation as described abovewith modulation elements which provide phase modulation only, which canbe addressed irrespective of each other with different value componentsper modulation element, e.g. according to the two-phase encoding method,has already been described by the applicant in document WO 2007/082707A1.

The above-mentioned publication shows a preferred way of encoding aspatial light modulator device with multiple phase values. A complexobject scanning value is represented by a sum of two phase componentswith same absolute amplitude value and different phase values andencoded to two adjacent phase-modulating light modulator cells of thesame modulation array. This means that each complex object scanningvalue with the phase ψ and amplitude a ranging between 0 and 1 is thuscomposed of the sum of two complex phase components with the sameamplitude value and the phase values ψ±acos a. It is also mentioned inthe international patent publication that the number of phase modulatorcells that constitute a modulation element is not necessarily limited totwo.

A spatial light modulator device which provides phase modulation onlyhas great advantages over light wave modulation with modulator cells foramplitude modulation. A light modulator device with two-phase encodingshows the reconstruction at greater brightness, because the modulatorcells realise maximum light transmittance with each phase setting.Another advantage of the two-phase encoding method is that it provides amore favourable wavelength dependence during reconstruction, whichallows colour video scenes to be reconstructed at high quality.

The mentioned two-phase encoding method is meant to achieve thesituation that those light wave portions of the light wave field whichis capable of generating interference which are modulated by adjoinedmodulator cells of a modulation element show the same opticalinterference effect as those light wave portions which are modulated bysuch a single modulator cell that is simultaneously addressed with allphase components of a complex object scanning value.

However, this is difficult to be realised because the modulator cellswhich are combined to form a modulation element lie side by side in themodulation array, thus having a spatial offset and showing differencesin the length of the optical path, also known as retardation, with amagnitude that depends on the kind of hologram, on the eye position ofan observer who sees reconstructed object light points, and—for examplewith Fourier holograms—on the position of the object light point whichis to be reconstructed by these modulator cells. This offset of themodulator cells effects phase differences among the modulator cells ofthe modulation elements, said phase differences depending on theposition of an observer eye and from the desired angular position of thereconstructed object point in respect of the optical axis of the system(depending on the kind of hologram) and impairing the quality of thereconstruction of the video scene, so that they require correction ofeach modulation element. In a holographic reconstruction system withposition controller and eye finder, which, as described above, track thepropagating modulated wave field optically upon a change in an observerposition, it is also very advantageous to have a tolerance in theobserver position of a few millimetres around the eye position which hasbeen detected by the eye finder. The occurring differences in theoptical path lengths would substantially restrict this slight freedom ofmovement of an observer in front of the holographic reconstructionsystem while watching a holographic reconstruction.

A solution to this problem has been suggested in the internationalpatent application WO 2008/132206 A1 titled “Light modulator forrepresenting complex-valued information”. According to this solution, astructured retardation layer made of a birefringent material is arrangedin the optical path of the modulation array, i.e. upstream and/ordownstream of the modulation array and preferably in close contact withthe modulator cells, said layer effecting an angular-position-specificadaptation of the optical path length of the emitted modulated lightwave portion to the lengths of the optical paths through the other cellsof the modulation element at least for one modulator cell of eachmodulation element. The layer thickness of this retardation layer ischosen such that the retardation layer counteracts theangular-position-specific change in the optical path among the modulatorcells of each modulation element by way of changing the optical pathlengths and compensates them at least partly. This solution has thedisadvantage that it requires a very finely structured but at the sametime rather thick layer.

The device disclosed in the unexamined application DE 2 058 418 titled“Device for determining the position of an object in an arbitrarysection of a pencil of rays” takes advantage inter al. of a point lightsource and a Savart plate. That Savart plate comprises two seriallyarranged birefringent uniaxial plate-shaped crystals which are arrangedsuch that their main sections are turned to a perpendicular situation,where the angle of the optical axis and the crystal surface is the samefor both crystals. The document teaches that the Savart plate dividesthe pencils of rays which are originally emitted by an point lightsource and which are incident on its entry side into twolinear-polarised partial pencils which show mutually perpendicularpolarisation and appear to origin in two linear-polarised virtual lightsources which lie on the entry side in a plane in symmetry with theoriginal light source. The document further teaches that there is nodifference in optical path length among partial pencils of rays whichshow mutually perpendicular polarisation in each point of the plane towhich the virtual light sources are arranged in mirror-symmetricalarrangement. In any other points, there are differences in optical pathlength among the partial pencils.

In the present invention, the term ‘Savart plate’ is used to designateany at least one single birefringent plate-shaped uniaxial crystal.Further, the birefringent material shall not be limited to conventionalcrystals such as quartz or calcite, but can also be generated forexample by an oriented polymer and/or a suitable polymer layer orsuitable film.

In the context of the present invention it is immaterial how themodulator cells are actually designed. It is for example possible to usea modulation array of liquid crystal cells or of electrowetting cells.The modulator cells can be of such nature that the modulation arraymodulates the light waves of the wave field during their passage or whenbeing reflected.

An alternative to a spatial light modulator device with phase-modulatinglight modulator cells can be a modulation array which exclusivelycomprises amplitude-modulating light modulator cells, where eachmodulation element is composed of multiple amplitude-modulating lightmodulator cells. An encoding method for a light modulator device whichcomprises two amplitude-modulating light modulator cells, namely one forthe real part and one for the imaginary part of a complex number, isknown as bias encoding. Another encoding method for a light modulatordevice which comprises three amplitude-modulating light modulator cellsis known as Burckhardt encoding.

A phase error which substantially impairs the quality of thereconstruction always occurs in a complex modulation element withmultiple adjoining modulator cells as a consequence of theangular-position-specific difference in the optical path lengths amongthe individual modulator cells, irrespective of the nature of themodulator cells of the modulation array.

SUMMARY OF THE INVENTION

It is thus the object of the present invention to provide compensationmeans which compensate the differences in the optical path lengths amongthe staggered modulator cells of one and the same modulation element ina spatial light modulator device comprising complex modulation elementswith multiple modulator cells which are arranged at laterally staggeredpositions. In addition, the compensation shall be at least widelyindependent of variations in the light wavelengths which are used formodulation, which occur for example as an effect of temperature changesin the light sources which serve to generate the light wave field.

A modulated light wave portion which leaves a modulation element shallhave the same effect in the holographic reconstruction as if the lightwave portion originated in a single compact modulator cell which can bemodulated with discrete complex hologram values. Light wave portionswhich are thus treated by the means of this invention cannot showinherent phase differences caused by differences in the optical pathlengths.

The present invention is based on a light modulator device whichcomprises at least one modulation array which is composed of individualor discretely encodable modulator cells. The modulator cells arecombined to form modulation elements. The modulator cells can modulatelight waves which are capable of generating interference of apropagating light wave field with holographic information in a spatiallystructured way. The modulator cells of each modulation element arearranged side by side in the modulation array in respect of thedirection of propagation of the propagating light wave field, and eachmodulation element can be encoded with a presettable or discrete complexobject scanning value in order to reconstruct or represent spatiallyarranged object light points holographically.

In order to circumvent said disadvantages, according to the presentinvention, in the light modulator device, the modulator cells of themodulation array are assigned with light wave multiplexing means withwhich the light wave portions which are modulated by the modulator cellscan be combined for each modulation element on the exit side by way ofrefraction or diffraction to form a modulated light wave multiplex suchthat the modulated light wave multiplex leaves the modulation elementsubstantially through a common position, i.e. substantially spatiallyoverlapped, and substantially in a common direction of propagation.

In the context of the present invention, ‘light wave multiplexing means’or ‘optical multiplexing means for space division multiplexing ofexiting light wave portions’ shall be understood in the present documentto be structured optical arrangements which deflect light wave portionswhich hit an entry-side interface of the optical arrangement atdifferent positions substantially in a parallel directed manner insidethe optical arrangement or component unit by a structure of wavedeflection elements such that at least certain light wave portions leavethe optical arrangement through a common exit position in an exit-sideinterface in a substantially common direction of propagation.

The optical multiplexing means for space division multiplexing ispreferably designed in the form of a flat optical plate unit which isarranged as close as possible to the modulation array and which has anareal structure of optical wave deflection means whose shape, size andposition are congruent to and match those of the modulator cells of themodulation elements and where at least for a part of the modulator cellsthe exit position of the light waves is arranged at an offset to theentry position of the light waves.

The desired spatial light wave multiplexing per modulation element isachieved in that at least some of the modulator cells of each modulationelement are assigned with wave deflection elements which have inside anoptical transfer axis which differs from the system axis of themodulation array, so that the light wave portions of all modulator cellsof each modulation element exit through said common exit position in anexit-side interface in a common direction of propagation. The opticalmultiplexing means realise an individual wave exit position for eachmodulation element.

The optical means for space division multiplexing can be structured flatoptical elements such as film arrangements which include volumeholograms, micro-prism arrays and/or birefringent optical elements whosestructure is matched to the shape, size and position of the modulatorcells of the modulation elements.

Polarisation Gratings as Beam Combiners

If light portions which pass through different modulator cells coverdifferent optical path lengths, it is generally necessary in order tomaintain a defined or desired light interference effect that thedifference in the optical path lengths between the portion of lightwhich passes through one modulator cell and the portion of light whichpasses through the other modulator cell is corrected with the help of anoffset phase. Additionally, there are temperature-fluctuation-inducedpath length variations among the light portions which pass throughdifferent modulator cells, as already mentioned above.

This can be achieved for example by an arrangement with symmetric lightdeflection as shown in FIG. 8. First the light which comes from the onemodulator cell, and in the next part of the arrangement the light whichcomes from another modulator cell (pixel) is deflected by half thedistance between modulator cell centres or pixel pitch. Such anarrangement requires either two Savart plates which are turned at anangle to each other or altogether four instead of two volume gratings(see FIG. 6 or 7) and, additionally, a polarisation-turning layerbetween the two Savart plates or between the two pairs of volumegratings, respectively.

Besides volume gratings, other types of grating structures are known aswell, for example polarisation gratings. They serve as diffractiongratings or for beam deflection at maximum efficiency in only one of thefirst orders (+1^(st) or −1^(st) order only), in contrast to other knowngratings which often show 50% efficiency in the +1^(st) and 50%efficiency in the −1^(st) diffraction order.

While 50% of linearly polarised light is deflected in the +1^(st) and50% in the −1^(st) diffraction order by a polarisation grating, it hasthe property of deflecting 100% of circularly polarised light in onlyone of those 1^(st) orders. In which order depends on whetherright-handed or left-handed circularly polarised light falls on them.

Further, achromatic polarisation filters are known as well whichcomprise a high diffraction efficiency for different wavelengths, whichis described in citation [1].

It is a further object of the present invention to realise a combinationof two phase pixels and an arrangement of a minimum number of gratingstructures such that both phase pixels have symmetrical beam paths, thusshowing more tolerance e.g. as regards thickness differences caused bythe temperature and other ambient conditions.

This is achieved in that, first, in contrast to known methods astructured λ/4 layer (instead of the λ/2 layer) is arranged at the pointof exit of the two phase pixels.

Through a different orientation of the λ/4 layers at the two modulatorcells or pixels, based on a linear polarisation at the point of exit ofthe modulation array or SLM, the light emitted by one of the two phasepixels of a modulation element (macro-pixel) is given left-handedcircular polarisation, while the other one is given right-handedcircular polarisation.

Secondly, polarisation gratings are used instead of volume gratings. Thepolarisation grating deflects light coming from the two phase pixels inopposite directions, because of their different circular polarisation.The light coming from both pixels then runs through a spacer—an elementthat is designed similar to the stack of volume gratings—and movestowards each other, but in this case symmetrically. Since both beams aredeflected, the spacer can be of thinner design than with volumegratings, which is another advantage over the latter. The polarisationof the light is straightened again by a second polarisation grating, orrather deflected from two different directions of propagation into acommon direction of propagation.

Subsequently, a polariser combines the superposed light to form acomplex value—the same step as with Savart plates or volume gratingstacks. However, this polariser has an orientation of the transmittancedirection which is turned by 45°, compared to that in an arrangementwith Savart plate or volume gratings, namely vertical or horizontal.

This can be described by a Jones matrix equation.

Right-handed circularly polarised light has a Jones vector which isproportionate to

$\begin{pmatrix}1 \\j\end{pmatrix}$

Left-handed circularly polarised light has a Jones vector of

$\begin{pmatrix}1 \\{- j}\end{pmatrix}$

If the light of the two phase pixels (modulator cells) has the phases φ1and φ2, respectively, their sum has the vector

$\begin{pmatrix}{e^{j{\varphi 1}} + e^{j{\varphi 2}}} \\{j\left( {e^{j{\varphi 1}} - e^{j{\varphi 2}}} \right)}\end{pmatrix}$

A horizontal polariser has the Jones matrix

$\begin{pmatrix}1 & 0 \\0 & 0\end{pmatrix}$

This means that downstream of the polariser a complex number

e ^(jφ1) +e ^(jφ2)

is realised, as is intended in two-phase encoding.

Alternatively, with a vertical polariser the complex number

e ^(jφ1) −e ^(jφ2)

would be represented.

This corresponds with the results that would be achieved for volumegratings, linearly polarised light and polarisers under +45° or −45°.

The beam combining shall be achieved in a colour display in particularfor red, green and blue light.

An achromatic grating as described in cit. [1] can be used. However, itis also possible to use a more simple grating, which is only optimisedfor one wavelength. Other wavelengths then suffer from diffractionlosses. However, the non-diffracted light can be blocked by apertures sothat it does not disturb the observer who looks at an holographicdisplay.

Further, a deflection angle which changes as the wavelength varies canbe compensated by apertures, as has been proposed in the context of thesolutions described above.

FIG. 9 illustrates the functional principle of a prior art polarisationgrating according to cit. [2]. The drawing shows a dynamic element.However, it is intended to use passive elements in the context of thisinvention.

FIG. 10 shows the beam path in a volume grating (non-symmetrical). PixelP01 is followed by a λ/2 plate with a first orientation, and pixel P02is followed by a λ/2 plate with a different orientation. FIG. 11 showsthe beam path in an arrangement with polarisation gratings(symmetrical). Pixel P01 is followed by a λ/4 plate with a firstorientation, and pixel P02 is followed by a λ/4 plate with a differentorientation.

FIG. 12 shows an exemplary arrangement: two pixels (encodable modulatorcells) P1, P2, which emit linearly polarised light (filled red arrows),are followed by a structured λ/4 layer QWP. The optical axis which isturned by +45° for the one pixel, P1, and by −45° for the other pixel,P2, to the direction of polarisation of the light which is emitted bythe SLM (modulation array) and through which circularly polarised lightis generated (indicated by red circular arrows) is shown in the diagram.

According to the embodiment shown in FIG. 12, a first polarisationgrating Pg1 deflects the light according to its polarisation. Once thelight has passed a spacer DL (thin glass plate or polymer film) ofsuitable thickness and once it is spatially superposed, it is deflectedin opposite directions by a second polarisation grating Pg2, so that thelight which comes from the two pixels leaves parallel. Downstream ofthose elements, a linear polariser Pol is arranged either at 0° or at90°.

Polarisation gratings have the property of changing the direction ofrotation of circular polarisation, i.e. from right-handed circularpolarisation to left-handed circular polarisation and vice versa (whichis also indicated in the drawing).

This property is very advantageous for the application as a beamcombiner, because it allows the use of two gratings of the same kind(i.e. with identical orientation of the molecules in the grating) in thearrangement.

Circular polarised light is deflected by the first grating, therebychanges the direction of rotation of its polarisation, and is thusdeflected in the opposite direction by the second grating of the samekind. Two gratings of the same kind which are arranged one after anotherin the optical path thus lead to the desired parallel offset.

According to a preferred embodiment, the light wave multiplexing meansinsofar comprises at least a polarisation means and a first and a seconddeflection layer Vg1 and Vg2. The polarisation means serves to assign tothe light which passes through a first modulator cell 1 a presettablefirst polarisation. The polarisation means further assigns to the lightwhich passes through a second modulator cell 2 a presettable secondpolarisation. The first deflection layer Vg1 is arranged downstream ofthe polarisation means, seen in the direction of light propagation. Thefirst deflection layer Vg1 is then followed in the direction of lightpropagation by the second deflection layer Vg2 at a defined distance d.The presettable first polarisation can be perpendicular to thepresettable second polarisation. Alternatively, the presettable firstpolarisation can be circular and have an opposite direction of rotationcompared to a presettable second circular polarisation. If the lightalready has a suitable structured polarisation, e.g. due to theproperties of the light source used, it is not generally necessary touse a polarisation means.

Referring to FIG. 7, the optical property of the first deflection layerVg1 is chosen such that the light which passes through the firstmodulator cell 1 is substantially not deflected while the light whichpasses through the second modulator cell 2 is deflected by a firstdefined angle. The optical property of the second deflection layer Vg2is chosen such that the light which passes through the first modulatorcell 1 is substantially not deflected while the light which passesthrough the second modulator cell 2 is deflected by a second definedangle. The absolute value of the second defined angle is substantiallyidentical to the absolute value of the first defined angle.

Referring to FIG. 8, the second deflection layer Vg2 is followed by athird and fourth deflection layer Vg3, Vg4 at defined distances in thedirection of light propagation. The optical property of the thirddeflection layer Vg3 is chosen such that the light which passes throughthe first modulator cell 1 is deflected by a third defined angle whilethe light which passes through the second modulator cell 2 issubstantially not deflected. The optical property of the fourthdeflection layer Vg4 is chosen such that the light which passes throughthe first modulator cell 1 is deflected by a further, fourth definedangle while the light which passes through the second modulator cell 2is substantially not deflected. The absolute value of the third definedangle can be substantially identical to the absolute value of the fourthdefined angle.

The polarisation means can comprise a retardation plate having aplurality of regions which are characterised by different orientations.This is particularly preferable where the functional principle of themodulator cells is already based on polarised light, or at least wheretheir function is not adversely affected by the use of polarised light.Otherwise, a structured polariser with multiple regions having differentorientations must be used in which light of a certain polarisationdirection is absorbed. However, this would be associated with a loss oflight. In this context, a structured polariser shall in particular beunderstood to be a polariser which comprises first spatial regions andsecond spatial regions which assign to the light which interacts withthe polariser certain presettable polarisations, where the first spatialregions are assigned to one class of modulator cells and the secondspatial regions are assigned to another class of modulator cells. Theretardation plate can be a λ/2 plate or a 1x+λ/2 or 1x−λ/2 plate, i.e.the retardation plate comprises a relative phase shift of λ/2.Alternatively, the polarisation means can comprise a first retardationplate having a first orientation and a second retardation plate having asecond orientation. The first and the second retardation plate can eachbe a λ/2 plate. The first retardation plate having the first orientationis then assigned to the light which passes through the first modulatorcell 1. The second retardation plate having the second orientation isassigned to the light which passes through the second modulator cell 2.

Referring to FIGS. 11 and 12, the optical property of the firstdeflection layer Pg1 is chosen such that the light which passes throughthe first modulator cell P01 is deflected by a first defined angle intoa first direction while the light which passes through the secondmodulator cell P02 is deflected by a second defined angle into a seconddirection. The optical property of the second deflection layer Pg2 ischosen such that the light which passes through the first modulator cellP01 is deflected by the second angle while the light which passesthrough the second modulator cell P02 is deflected by the first angle.The absolute value of the first angle can be substantially identical tothe absolute value of the second angle.

The polarisation means can comprise a retardation plate having aplurality of regions which are characterised by different orientations.This is particularly preferable where the functional principle of themodulator cells is already based on polarised light, or at least wheretheir function is not adversely affected by the use of polarised light.Otherwise, a structured circular polariser with multiple regions havingdifferent orientations must be used. However, this would be associatedwith a loss of light. The retardation plate can be a λ/4 plate or a1x+λ/4 or 1x−λ/4 plate, i.e. the retardation plate comprises a relativephase shift of λ/4. Alternatively, the polarisation means can compriseat least a first retardation plate having a first orientation and asecond retardation plate having a second orientation. The first and thesecond retardation plate can each be a λ/4 plate. The first retardationplate having the first orientation is in this case assigned to the lightwhich passes through the first modulator cell P01. The secondretardation plate having the second orientation is assigned to the lightwhich passes through the second modulator cell P02.

A deflection layer Vg1, Vg2, Vg3, Vg4, Pg1, Pg2 can be a layer thatcomprises a hologram and/or a volume grating and/or a Bragg grating, ora polarisation grating.

A polarisation means WGP, Pol with presettable optical property whichhas the effect of an analyser can be arranged downstream of thedeflection layer Vg1, Vg2, Vg3, Vg4, Pg1, Pg2 in the direction of lightpropagation.

In all embodiments of the present invention, an apodisation element APFcan be provided which affects the light beams of a modulation element MEwhich have been combined to form a modulated light multiplex. Theapodisation element APF can comprise in a direction transverse to thedirection of light propagation a neutral intensity profile which issubstantially independent of the respective wavelength of the usedlight. Such an intensity profile can be described by an analyticallywritable apodisation function, e.g. a cosine or triangular or Blackmanor Hamming or Welch window function. Specifically, the apodisationelement APF can have corresponding apodisation masks each of which beingassigned to one modulation element ME. Such an apodisation mask, forexample as shown in FIG. 16 on the left in a side view, then affects themodulated light wave multiplex of combined light beams of thatmodulation element ME. The apodisation mask can for example be arrangeddownstream of the polariser WGP, which serves as analyser, that is atthe position denoted by PC in FIG. 13.

For colour applications, an adequately designed apodisation element APFCcan be provided which affects the light beams of a modulation element MEwhich have been combined to form a modulated light multiplex. Theapodisation element APFC has at least two intensity profiles which aredependent on the respective wavelength of the used light. The intensityprofiles are mutually shifted by a presettable value laterally in adirection transverse to the direction of light propagation. This isshown in FIG. 16 on the right in a side view. The intensity profiles canbe contained in individual layers APFSR, APFSG, APFSB which are arrangedone after another in the direction of light propagation.

Manufacture of Passive Layers

Citations [1] and [2] describe actively switchable LCPG.

They are manufactured such that orientation layers of aphoto-polymerisable material are exposed to UV radiation. Two UV lightsources with opposite circular polarisations are used and their light issuperposed. The grating constant is set through the relative angle atwhich the light sources are superposed. If substrates with adequateorientation layers exist, an LC layer whose thickness is for exampledefined by spacer balls is filled in between the substrates.

Other LC materials are known from other applications which arecross-linked on a substrate after having been oriented, so that theirorientation is quasi frozen. As regards the application as beamcombiners, passive LCPG are preferably used. Therefore, the use ofpolymer materials is proposed here.

Beam Combiner for RBG

There are two different effects with view to the wavelength of the usedlight which need to be considered.

(a) The diffraction efficiency of a grating generally changes with thewavelength. This effect typically depends on the thickness of thegrating.

(b) The diffraction angle also generally changes with the wavelength. Itis dependent on the ratio of wavelength to grating constant.

Re (a) Diffraction Efficiency:

Prior art citation [1] describes a special polarisation grating whichcomprises a high diffraction efficiency throughout the entire visiblerange. However, that grating still has different angles for red, greenand blue light.

It is further described in the prior art to change the effectivebirefringence in an active LCPG by respectively controlling andpartially orienting the LC molecules in the array such that the equationd Δn_(eff)(V)=λ/2 is optionally satisfied for different wavelengthsdepending on the applied voltage.

This also raises the diffraction efficiency but does not change thediffraction angles. That element can be used as a beam combiner in aholographic display with time division multiplexing of colours, wherethe grating is adapted to the actually processed colour by applying arespective voltage. An active grating assumes that the grating itself isaddressed and this addressing is synchronised with the control of thelight sources and SLM.

For a spatial multiplexing of colours there would be the possibility toapply different voltages to the LC material to individual pixel columnsduring the manufacturing process and to polymerise it in that state.

Re (b) Diffraction Angle:

As regards the use as a beam combiner, great importance is in particularattached to achieving same diffraction angles for red, green and bluelight. The above-mentioned approaches do not satisfy this requirement.

A preferred possibility of achieving identical diffraction angles is toperform space division multiplexing of the grating periods in agreementwith a space division multiplexing of the colours of the SLM. For this,a mask is used during the exposure of the orientation layers of thesubstrates (see Section “Manufacture of passive layers” above), saidmask covering in stripes about ⅔ of the area, namely that part whichcorresponds with the colour pixels of the two other colours. The angleof the two exposing UV light sources is then adapted such to achieve adesired grating constant for one colour (RGB). This process is repeatedthree times with the mask shifted and the angle altered accordingly.

In contrast to Bragg gratings, where multiple gratings can be superposedor arranged one after another in series arrangement, three interleavedgratings are obtained here which are not superposed.

A combination of the prior art grating structure according to citation[1] for a high diffraction efficiency for all wavelengths can becombined with this embodiment for an identical diffraction angle of theindividual wavelengths.

Alternatively, the method can also be used on its own if a highdiffraction efficiency is to be achieved for one wavelength and forother wavelengths the non-diffracted light is filtered out otherwise sothat it does not get though to the observer. This can be done by takingadvantage of exit position, exit angle or, as the case may be,polarisation of that light.

Numeric Examples

According to the prior art, the ratio of layer thickness to gratingconstant is limited in polarisation gratings. This limit also depends onmaterial properties of the LC, e.g. its birefringence.

Since the condition d Δn=λ/2 must be satisfied for the layer thickness d(where Δn is the refractive index difference and λ is the wavelength),there is a minimum grating constant and, consequently, a maximumdeflection angle.

Gratings with a grating constant of about 6 μm have already beendescribed in experimental setups. Theoretical limits are likely to bearound 2 μm. The consequent deflection angles are about (2 times) 5degrees.

It can be assumed that the arrangement of grating+spacer+gratingtypically has a thickness of about ½ to ⅓ of that of a Savart plate madeof a material with same refractive index difference Δn. However, in sucharrangement, the polarisation grating itself is only few micrometres,typically 2-3 μm, thick. The spacer would have a thickness rangingbetween 200-300 μm (with a grating constant of about 4-6 μm) for a pixelpitch of 60 μm.

Polarisation Grating Stack

Another possibility is to use a stack of multiple polarisation gratingswhich are arranged one after another instead of a single polarisationgrating. Polarisation gratings are sensitive as regards the angle ofincidence.

However, when using passive gratings with a fix total deflection angle,it is possible to optimise subsequently arranged gratings in respect ofa successively larger angle of incidence.

The deflection angle can thus generally be increased and the totalthickness reduced in that multiple polarisation gratings are arranged inseries arrangement.

Achromatic Refractive Beam Combination

As has been shown above, there is a refractive solution (Savart plate)and a diffractive solution (volume gratings) to the object of combiningtwo phase-shifting pixels (modulator cells) to form a resultantsecondary pixel (modulator element) which generates complex values, i.e.which is capable of modulating or varying both phase and amplitude ofthe light that passes through those pixels.

It is a particular purpose of this document to propose an achromaticoption of the refractive solution in the context of sequential colourrepresentations.

FIG. 13 shows the generation of a complex-valued pixel with the help oftwo phase pixels. The drawing shows the Savart plate SP, which iscombined with a structured half-wavelength plate λ/2 and a polariser WGPto generate a complex-valued pixel PC. The exemplary cosine-shapedapodisation profile of the resultant pixel is not shown in the drawing,i.e. the pixel is shown as a uniformly transparent pixel. The refractiveindex difference between ordinary and extraordinary axis is Δn_(oe)=0.2,which corresponds to a deflection angle of the TM polarised light ofα_(TM)=7.384° and a 0.1296 μm beam offset per micrometer platethickness. FIG. 13 shows these relations to scale.

In a birefringent material, the extraordinary beam propagates at acertain relative angle to the ordinary beam. However, at the point ofexit of the birefringent material into an optical isotropic medium,ordinary and extraordinary beam are directed to be parallel again. Lightof a certain polarisation is thus given a deflection by an angle thatdepends both on the magnitude of birefringence and the orientation ofthe optical axis of the birefringent material at the entry-sideinterface of the birefringent material, and a deflection in the oppositedirection at the at the exit-side interface. A parallel offset is thusobserved whose magnitude depends on the thickness of the birefringentbody. This effect is particularly well observed if the birefringentmaterial has the form of a coplanar plate.

FIG. 14 illustrates the problem of dispersion in sequentialrepresentations. The drawing shows the Savart plate SP which, caused bythe dispersion n=n (λ) for the red and blue wavelength, bears an errorΔs in the lateral offset s. If in the Savart plate SP the design anglewhich lets the beam of the green wavelength lie centrally on the forexample cosine-shaped apodisation profile (not shown) is used for theTM-polarised beam, then a larger angle is obtained for the blue beam anda smaller angle is obtained for the red beam, thus causing a positive ornegative beam offset, respectively, i.e. a beam offset towards the oneor respective other side of the apodisation profile.

In the context of a sequential colour representation, the problem isthat the apodisation filter of an individual complex-valued pixel is notilluminated centrally by two colours, i.e. red and blue in this example.

A simple solution would be to reduce the dimensions of the apodisationprofile, i.e. to cut the fill factor FF from 0.8 to 0.6, for example.However, this means to cut off almost 50 percent of the transparentarea, or to eliminate 50 percent of the radiant emittance throughabsorption. Moreover, the efficiency of the suppression of diffractionorders neighbouring the observer window is trimmed down when the fillfactor is reduced.

Another solution is to perform space division multiplexing of thecolours used, i.e. to use a spatially structured arrangement of colourfilters, which is unproblematic for one-dimensionally encoded 3Dobjects, that is for horizontal parallax only (HPO) holograms, forexample. This is a practicable way if enough pixels are available toprovide a 3D reconstruction that goes clearly below the resolvingcapacity of the human eye. This is illustrated in FIG. 15, namely in theform of space division multiplexing of phase pixels which are combinedto form complex-valued pixels. For example, the first column S1R on theleft has a red filter. The column S2B on the right of the latter (withan absorbing black column SB in between) has a blue filter. The columnS2G on the right of the latter (with an absorbing black column SB inbetween) has a green filter. This arrangement continues periodically.

Another solution is to use a colour-selective apodisation filterdistribution instead of a ‘neutral density apodisation profile’, i.e. anintensity or transmittance filter profile in the form of a distributionof greyscale values. This is shown in FIG. 16, namely in the form of atransition from a greyscale-value-or neutral-density-type apodisationfunction (left, filter denoted by APF) to a laterally offsetcolour-selective apodisation function (right, filter denoted by APFC).In analogy with the layer structure of reversal films for colourtransparencies or slides, a colour-selective filtering can for examplebe achieved in subsequently arranged layers APFSR (red), APFSG (green)and APFSB (blue). To solve the problem, the points of maximumtransmittance depend on the respective spectral colour. However, thefill factor can be the same for all colours. Moreover, colour-selectivemodifications of the apodisation function can be conducted e.g. in orderto spectrally optimise the energy efficiency or the suppression ofdiffraction orders neighbouring the observer window.

Referring to FIG. 16, the centre of the intensity distribution is shownto have a spectrally different lateral offset which cannot be perceivedin the holographic reconstruction with a holographic direct-viewdisplay, because it is smaller than for example 10 μm.

Yet another solution is to use at least two different birefringentmaterials SP1, SP2, which show different, i.e. normal and anomalousdispersion. This means that the Savart plate is made up of two layersSP1, SP2, where for example the first layer SP1 has the highestrefractive index for the blue spectral line and the lowest refractiveindex for the red spectral line, while the second layer SP2 has thelowest refractive index for the blue spectral line and the highestrefractive index for the red spectral line.

The thickness ratio of the two plates SP1, SP2 is proportionate to theratio of the refractive index differences to the green spectral line.The plate thicknesses can be chosen such that the squared lateralposition deviation is minimised over all spectral colours. Such achromatically corrected Savart plate which is composed of the two platesSP1, SP2, is shown in FIG. 17.

According to the embodiment described above, the light wave multiplexingmeans comprises at least one polarisation means and at least onebirefringent medium SP with presettable optical property. Thepolarisation means serves to assign to the light which passes through afirst modulator cell P01 a presettable first polarisation. Thepolarisation means further assigns to the light which passes through asecond modulator cell P02 a presettable second polarisation. Thebirefringent medium SP is arranged downstream of the polarisation meansand/or the first and second modulator cell P01, P02, seen in thedirection of light propagation. The presettable first polarisation canbe perpendicular to the presettable second polarisation.

The optical property of the birefringent medium SP is chosen such thatthe light which passes through the first modulator cell P01 issubstantially not deflected by the birefringent medium SP, while thelight which passes through the second modulator cell P02 is deflected bya first defined angle at the entry-side interface of the birefringentmedium SP. The light which passes through the second modulator cell P02is deflected by a second defined angle at the exit-side interface of thebirefringent medium SP which is coplanar with its entry-side interface.The absolute value of the first angle can be substantially identical tothe absolute value of the second angle. The light which passes throughthe second modulator cell P02 thus leaves the birefringent medium SPsubstantially at a parallel offset after its passage through thebirefringent medium SP.

In FIGS. 6 to 11, 12 to 14, 17, 20 and 21, the optical properties of thedeflection layers Vg1, Vg2, Vg3, Vg4, Pg1, Pg2 and, if any, of theprovided retardation plates and/or the optical properties of thebirefringent media SP, SP1, SP2, SP3, SV1, SV2 are chosen such that thebeam deflection, if any, is oriented in a direction which substantiallylies in the drawing plane of the respective figure. However, otherconfigurations of the optical properties of involved components arepossible, where beams may also be deflected in a direction which isoriented out of the drawing plane of the respective figure. Insofar, amodulated light wave multiplex of a modulation element does not onlyleave the light wave multiplexing means with a lateral offset in onedirection (e.g. along a column of modulator cells), but with a lateraloffset in a first and in a second direction.

The birefringent medium SP1 with a normal or anomalous dispersion can befollowed in the direction of light propagation by another birefringentmedium SP2 with an anomalous or normal dispersion, i.e. a respectivelyopposite dispersion. This is shown in FIG. 17. The thickness ratio ofthe two birefringent media SP1, SP2 is then presettable and preferablydepends on the refractive index difference ratio of the two birefringentmedia SP1, SP2 and on the ratio of a presettable wavelength of thelight, e.g. green, and at least one further presettable wavelength ofthe light, e.g. red and blue.

Beam combination in a way similar to that shown in FIG. 8 is alsopossible when using at least one birefringent medium. This is shown inFIG. 21. For this, the birefringent medium SP1 can be followed in thedirection of light propagation by another birefringent medium SP3. Theoptical property of the further birefringent medium SP3 is chosen suchthat the further birefringent medium SP3 deflects the light which passesthrough the first modulator cell P01 by a defined third angle at theentry-side interface of the further birefringent medium SP3 and by adefined fourth angle at the exit-side interface of the furtherbirefringent medium SP3, which is coplanar with its entry-sideinterface. The further birefringent medium SP3 does substantially notdeflect the light which passes the second modulator cell P02. Theabsolute value of the third defined angle can be substantially identicalto the absolute value of the fourth defined angle. The light beams whichenter the first birefringent medium SP1 are substantially given aparallel offset when they leave the second birefringent medium SP3. Aλ/2 layer is arranged between the two birefringent media SP1 and SP3,said layer turning the direction of polarisation of the light whichpasses through this layer by 90 degrees. The optical axes of SP1 and SP2(indicated by double arrows) are oriented at right angles.

A polarisation means WGP with presettable optical property which has theeffect of an analyser can be arranged downstream of the birefringentmedium SP; SP1, SP2 in the direction of light propagation.

BRIEF DESCRIPTION OF THE DRAWINGS

Now, there are a number of possibilities for embodying and continuingthe teachings of the present invention. To this end, reference is madeon the one hand to the dependent claims that follow claim 1, and on theother hand to the description of the preferred embodiments of thisinvention below including the accompanying drawings. Generally preferredphysical forms and continuations of the teaching will be explained inconjunction with the description of the preferred embodiments of theinvention and the accompanying drawings. The Figures are schematicdrawings, where

FIG. 1 shows a detail of spatial light modulator device of an embodimentaccording to this invention,

FIG. 2 shows a first embodiment of the optical multiplexing means forspatial multiplexing of exiting modulated light wave portions with anarray of micro-prisms and a volume grating,

FIG. 3 shows a second embodiment of the optical multiplexing means forspatial multiplexing of exiting modulated light wave portions with anarray of micro-prisms and a volume grating, where diffracted light isused,

FIG. 4 shows a third embodiment of the optical multiplexing means forspatial multiplexing of exiting modulated light wave portions, wherediffracted light is used and where the non-diffracted light is filteredout by a spatial frequency filter with an aperture mask,

FIG. 5 shows a fourth embodiment of the optical multiplexing means forspatial multiplexing of exiting modulated light wave portions with apolarising light wave splitter,

FIGS. 6 to 8 show embodiments of the optical multiplexing means with apolarising beam splitter which is compensated in respect of changes inthe light wavelengths,

FIG. 9 illustrates the functional principle of a prior art polarisationgrating according to cit. [2],

FIG. 10 shows the beam path in a volume grating (non-symmetrical),

FIG. 11 shows the beam path in an arrangement with polarisation gratings(symmetrical),

FIG. 12 shows an embodiment of the present invention,

FIGS. 13 and 14 each show a further embodiment of the present invention,

FIG. 15 illustrates space division multiplexing of phase pixels whichare combined to form complex-valued pixels,

FIG. 16 shows a ‘neutral density’ apodisation function (left) and alaterally offset, colour-sensitive apodisation function (right), and

FIGS. 17 to 22 each show a further embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Since all optical multiplexing means for each of the modulator cells ofthe modulation array have the same structure, only a single modulatorcell of the modulation array will be shown in the drawings referred tobelow in order to keep the drawings simple and comprehensible.

For the same reason, the optical multiplexing means will be describedbelow with the example of a modulation array with regularly structuredmodulator cells, where each modulation element comprises two adjacentmodulator cells of the modulation array. A typical example of such aspatial light modulator device is a spatial phase-modulating lightmodulator designed to implement the above-mentioned two-phase encodingmethod. Generally, the structure can also correspond to a modulationelement which comprises more than two modulator cells.

The following embodiments can also be adapted in a comparable way toamplitude modulation. In the latter case, a phase-shifting optical layerwould additionally be required for at least one modulator cell permodulation element. If the bias encoding method is employed, a fix phaseshift of π/2 is required for one of the two modulator cells, and if theBurckhardt encoding method is employed, phase shifts of 2π/3 and 4π/3are required for two of the three modulator cells.

FIG. 1 shows a modulation element ME with a first modulator cell P01 anda second modulator cell P02, both of which being arranged next to eachother in a modulation array. A light wave field LW which is capable ofgenerating interference illuminates the modulation element ME in themodulation array. A modulator control unit CU encodes each modulatorcell P01, P02 with a phase component of a complex hologram value, sothat each modulator cell P01, P02 emits a discretely modulated lightwave portion LWP₁ and LWP₂, respectively, with parallel optical axesa₀₁, a₀₂ in a direction D in order to generate a holographicreconstruction. According to this invention, an array of opticalmultiplexing means is arranged as close as possible to the modulatorcells P01, P02. The optical multiplexing means comprise a structure ofwave deflection means U1, U2, which are spatially assigned to themodulator cells P01, P02. The wave deflection means U1, U2 have opticalaxes which differ from each other and which are oriented in respect ofeach other such that the light wave portions LWP₁ and LWP₂, which comefrom the same modulation element ME, are combined in the array ofoptical multiplexing means and form a wave multiplex of a modulatedcommon light wave portion LWP₀ with a common optical axis a₀.

According to a preferred embodiment of the present invention, the arrayof optical multiplexing means comprises an optical plate unit of stackedoptical plates. The optical plates can for example comprise multipletransparent polymer layers with a presettable optical property—inparticular birefringence.

FIG. 2 shows a first embodiment of such a plate unit, which comprises amicro prism array PA which provides for the modulator cells P01, P02 ofeach modulation element ME a micro-prism which realises a desiredoptical wave deflection function for the modulator cells P01, P02. Thisoptical plate unit also combines the light wave portions LWP₁ and LWP₂of the modulation elements to form a wave multiplex of a modulatedcommon light wave portion LWP₀. This is achieved in that a volumehologram BG, also known as Bragg hologram, is additionally arranged inthe optical path of the optical plate unit. This volume hologram BG hasthe task of preventing an intersection of the propagating light waveportions LWP₁ and LWP₂ and to lead the two light wave portions LWP₁ andLWP₂, which have been modulated by the modulator cells P01 and P02 of amodulation element, into direction D without any difference in theoptical path lengths. The volume hologram BG is encoded such that isdirects light waves with defined wavelengths tightly in a greatlylimited deflection angle or exit angle. Any light wavelengths which arerequired for a colour reconstruction must be considered as definedwavelengths, e.g. the colours red, green and blue.

FIG. 3 shows a second embodiment of the optical plate unit of FIG. 2.The two embodiments differ in the provision or angle of incidence of thelight wave field LW which is capable of generating interference. In theembodiment according to FIG. 3, the light which is capable of generatinginterference hits the spatial light modulator device or the modulatorcells P01, P02 at an oblique angle to the optical axis, so that—as aconsequence of the oblique angle of incidence—the first diffractionorder can be used for reconstruction. In the embodiment according toFIG. 2, the light wave field which is capable of generating interferencehits the spatial light modulator device parallel with the optical axis,so that the zeroth diffraction order can be used for reconstruction.

Referring to FIG. 4, an additional telescopic filter array (TFA) with anaperture mask AP between two afocally arranged lens arrays systems L1,L2 allows to suppress undesired light portions, e.g. those ofneighbouring spatial diffraction orders in respect of the direction ofincidence of the light wave field of the 0th diffraction order or ofunused periodicity intervals. At the same time, the two afocallyarranged lens arrays systems L1, L2 allow the fill factor of themodulator cells of the modulation element ME in the modulation array tobe raised due to an optical magnification.

FIG. 5 shows another embodiment of the present invention, where apolarising light wave splitter Pol combines the light wave portions ofeach modulation element. The optical multiplexing means for spatialmultiplexing use a plate with polarisation elements Spol and Ppol, whichassign to each light wave portion of a modulator cell P01, P02 in themodulation element a discrete light polarisation, combined with abirefringent coplanar plate BP, which assigns to all modulated lightwave portions LWP₁, LWP₂ of a modulation element a discrete inclinedoptical axis. The optical axes of all light wave portions are inclinedin respect of each other and the strength of the coplanar plate BP ischosen such that all light wave portions are superposed at its exit-sideinterface.

A polarising light wave splitter, as shown in FIG. 6, is very sensitiveto changes in the wavelength which is chosen to generate the holographicreconstruction. A lateral offset which is dependent on the wavelength ofthe used light and a change in the phase relation of the light areobtained.

FIGS. 6 and 7 show two embodiments which illustrate the fundamentals ofrealising a self-compensating beam splitter double plate according toFIG. 8. Vg1 and Vg2 denote volume gratings which serve as beamsplitters.

The distance d between the two parallel grating planes must bed=a/(2·cos(π/2)), i.e. 0.57735 μm per μm modulator cell width, in orderto achieve a complete superposition of the light wave portion TE ofmodulator cell 1 and light wave portion TM of modulator cell 2, both ofwhich having the width a, downstream of the planar polarising beamsplitter Vg2.

Assuming 50 μm wide modulator cells, a thickness d=28.87 μm can beachieved with the 0°/60° geometry of the polarising beam splitters,while in comparison with that a Savart plate must have a minimumthickness of 385.8 μm if Δn=0.2 is to be obtained.

The pointing vectors of the polarised light wave portions TE and TM willbe parallel downstream of the polarising beam splitter if they wereparallel upstream of the polarising beam splitter. The parallelism ofthe exiting beams should therefore not be a problem here.

However, wavelength fluctuations of the light are problematic. Given amodulator cell width of 30 μm and, consequently, a thickness of thepolarising beam splitter double plate of 17.32 μm, a wavelengthdeviation of Δλ=1 nm will result in a relative phase difference betweenthe two superposed modulator cells of about 2π/10. In order to solvethis problem, it is possible to choose a polarising beam splittergeometry with lower diffraction angle.

A possible polarising beam splitter deflection geometry (with polarisingbeam splitters Vg1, Vg2) is 0°/48.2°, as shown in FIG. 6. The light waveportion TE is deflected, while the light wave portion TM is notdeflected in this example. FIG. 7 illustrates a 0°/41.2° polarising beamsplitter (Vg1, Vg2) which transmits TE-polarised light withoutdeflection, while it diffracts or deflects TM-polarised light.

Given a modulator cell width of a=50 μm and a maximum permitted distanceto a plane of EW prisms (not shown), as regards cross-talking amongneighbouring modulator cells, of D_(max)=5×a=250 μm, it followsθ_(min)=arctan(a/D_(max))=arctan(0.2)=11.31°. At Δn=0.2, the Savartplate achieves about 7.4°.

Since the possible polarising grating beam splitter geometries aremathematically terms of a series, there are also usable angles in arange around 11°. The required refractive index variation is then veryhigh though, i.e. usage of 11° as polarising beam splitter geometry isdeemed rather unrealistic.

However, a polarising beam splitter geometry of 0°/33.557° is notunlikely to be realised in practice, where there must still be arefractive index variation reserve for RGB multiplexing. At a stabilityof the wavelength of Δλ=1 nm, this geometry would correspond with arelative phase difference of the combined modulator cell beams ofΔφ<27/20.

There are a number of possibilities to compensate the effect of apossibly drifting key wavelength.

One possibility is to use the generated summed signal of the twocombined modulator cells in order to compensate the phase shift simplyand easily during operation. For this, the phase of one modulator cellcan for example be shifted such that as a result a certain targetintensity is achieved. This produces a value for a corrective phase tobe introduced.

Further, it is possible to introduce a set of phase shifts, i.e. atleast three, in order to determine with the help of phase-shiftinginterferometry the relative phase of the combined modulator cell with anaccuracy of <27/512.

In displays which only comprise few light sources, it presents itself touse two diodes per wavelength and light source which have spectrallydifferent characteristics. If the characteristic lines are known, thenthe wavelength can be determined with an accuracy of <0.1 nm from thesignals of the diodes. This principle is for example employed in thewavelength measuring device WaveMate™ supplied by the company Coherent.

When the key wavelength is known, the relative phase to be set in thecombined modulator cells can be corrected directly if the key wavelengthdrifts. This should result in a remaining error of <2π/256 when settingthe relative phase in the combined modulator cells.

The above-mentioned approaches for online correction can be combinedwith each other in order to improve the measurement accuracy and thus tocompensate the effect of a wavelength drift. Irrespective of that, alaser can also be stabilised to Δλ<0.1 nm.

FIG. 8 shows an embodiment of a compensated polarising beam splitter.Here, TM-polarised light is diffracted and TE-polarised light is notwith the first two beam splitters Vg1 and Vg2, which compares to thearrangement shown in FIG. 7. Further, TE-polarised light is diffractedand TM-polarised light is not with the other two beam splitters Vg3 andVg4, which compares to the arrangement shown in FIG. 6. The distancebetween the individual beam splitters Vg1 to Vg4 can here be less thanin the embodiments shown in FIGS. 6 and 7, because only half the beamoffset must be achieved for the TE- and TM-polarised light.

FIG. 8 further shows how a compensation of a drift of the lightwavelength can be achieved. The compensation of Δφ_(rel)(Δλ) is based onthe fact that this effect is equally distributed over the combinedmodulator cells. Since the required retardation layers (one structuredand one plane, unstructured) are only about 1.5 μm thick, the resultantdistance between SLM and polariser WGP is DD<2a (applies to a ˜50 μm),even if the thickness of the volume gratings Vg1, Vg2, Vg3 and Vg4,which are arranged in four planes, cannot be neglected (each about 10 μmthick). If a=70 μm, then the resultant thickness DD<a. If a=20 μm, DD isless than 3a, which makes it still possible for small modulator cells touse polarisers other than wire grid polarisers.

Referring to FIG. 6, a λ/2 plate is drawn between the modulator cell 2and the volume grating Vg1. Providing a λ/2 plate becomes necessary whenthe light which falls on the modulator cells 1, 2 only has onepresettable polarisation, e.g. a linear TE polarisation. In that case,the polarisation of the light which passes through the modulator cell 2is turned by the λ/2 plate by 90 degrees, so that the light which passesthrough the modulator cell 1 is given a polarisation that isperpendicular to that of the light which passes through the modulatorcell 2. If the light which falls on the modulator cells 1, 2 already hasa perpendicular polarisation, the provision of a λ/2 plate between themodulator cell 2 and the volume grating Vg1 is not necessary. In otherwords, it is important that the light which passes through the modulatorcell 1 has a different—e.g. perpendicular—polarisation than the lightwhich passes through the modulator cell 2, so that the light whichpasses through one modulator cell is deflected by the volume gratingVg1, and the light which passes through the other modulator cell is notdeflected by the volume grating Vg1. What has been said above alsoapplies to FIGS. 7, 8, 13, 14 and 17 in a similar way.

The angular geometry does not have to be met with an accuracy of <0.05°.An error of 0.1° is uncritical. If D<a, angle errors of 0.3° areuncritical, i.e. even if a measurement is necessary to compensate theresultant effects. This is due to the fact that the portion ofnon-diffracted light is blocked in the plane of the apodisation filterAPF if D<a.

However, it is necessary or at least recommended to accept the sandwichpart which comprises two volume gratings of like geometry as such. Alateral offset of the wave fronts which are to be superposed isunproblematic, because the fill factor of the apodisation filter APF issmaller than the fill factor of the phase-modulating SLM, i.e.FF_(APO)<FF_(SLM). The dimension of the light-emitting area is thusconstant, this area is sufficiently homogeneously illuminated andcomprises only common superposed wave front portions, i.e. a lateraloffset of as much as 5% would be unproblematic. In other words, alateral offset of the light wave portions which leave the modulationelement ME can be compensated with the help of a filter or shutter, e.g.an apodisation filter APF with a defined transmittance characteristic oran aperture mask with a defined mask geometry, which is arrangeddownstream of that modulation element ME. This can also be applied tothe embodiments according to FIGS. 5 to 7.

Refractive Beam Combination with Lenses or Prisms

Now, another possibility of refractive beam combination will bedescribed which is based on the use of lenses and/or prisms, orlenticulars and/or prism arrays.

FIG. 18 illustrates an embodiment and shows in a top view a detail of anoptical system 100 which comprises a lenticular L and a prism array P. Alens 102 of the lenticular L and a prism 104 of the prism array P areboth assigned to two pixels of the SLM (not shown in FIG. 18). Thedrawing shows the beams 106, 108 coming from two mutually assignedpixels, a lens 102 of the lenticular L and a prism 104 of the prismarray P. The pixel pitch is denoted by p, the diameter of one beam 106,108 upstream of the lens 102 is denoted by a, and the distance betweenlenticular L and prism array P is denoted by d.

Lens 102 focuses the light of each beam 106, 108 and converges the twobeams 106, 108. The distance d is somewhat smaller than the focal lengthof lens 102, so that the focused beams 110, 112 are a small distance Dapart in the plane of the prism array P. The two beams 110, 112 hitdifferent sides of a prism 104. The prism angle is chosen such that thebeams 114 substantially run in the same direction downstream of theprism. The drawing shows the double angle of divergence 20 and thedouble beam waist 2 w.

In this arrangement the two beams 106, 108 do not fully converge, butremain a narrow distance D apart. However, this distance is much smallerthan the original distance, which equals the pixel pitch. Therefore, thedifference in the optical path length of the light towards the edge of adiffraction order is much smaller, which greatly improves thereconstruction quality.

Now, a numerical example will be provided under the simplifyingassumption that the distance d between lenticular L and prism array Pequals the focal length f, i.e. d=f. Further, the beams are assumed tobe Gaussian beams. The pixel pitch is p=50 μm. The distance of the beamsis to be minimised from p=50 μm to D=p/10=5 μm. The beam waist is chosensuch that D=2·w.

The following relations apply:

Θ*w=λ/π (beam parameter product of a Gaussian beam=ratio of divergenceand beam waist of a Gaussian beam)

a=2Θ*f

D=2w

If with the help of this arrangement the distance p of the beams isreduced from 50 μm to 5 μm, then a focal length f=0.31 mm will resultfor a wavelength of 500 nm. The radius of the lenses would thus be about0.15 mm at a lens pitch of 0.1 mm.

Lenticulars L and prism arrays P are optical components which can bemade and aligned in large sizes. They serve to substantially reduce thedistance of the two beams 106, 108 and thus to improve thereconstruction quality.

FIG. 19 illustrates another embodiment and shows in a top view a detailof an optical system 100 which comprises two prism arrays P1 and P2 anda spacer glass plate G with a thickness d. The drawing shows two beams106, 108 coming from mutually assigned SLM pixels (not shown), saidbeams comprising perpendicular polarisation directions after havingpassed a structured retardation plate (not shown).

The first prism array P1 is made of an isotropic material. In contrast,the second prism array P2 is made of a birefringent material. Onedirection of polarisation is transmitted as an ordinary beam 108, 112without being deflected, while the perpendicular direction ofpolarisation is deflected as an extraordinary beam 106, 110. This issimilar to the birefringent lenticulars which are used by the companyOcuity for switchable 2D/3D displays. The ordinary refractive index ischosen to be equal to the refractive index of the surrounding material.In contrast, the extraordinary refractive index is chosen to bedifferent, so that the extraordinary beam is deflected.

The lower beam 108 passes through the prism array P1 without beingdeflected, because it hits the planar interface. On entry into thespacer glass plate G it is denoted by 112; it is not deflected by theprism array P2 either, because it has the direction of polarisation ofan ordinary beam. The upper beam 106 is deflected by both prism arrays,P1 and P2, because it is the extraordinary beam. Both beams 106, 108 arethus combined and leave the optical system in the form of a superposedlight beam 114 in the same direction.

Now, a numerical example will be given with a pixel pitch of p=50 μm.The thickness of the glass plate is assumed to be d=500 μm. In thisarrangement, the upper beam 106 must be deflected in each prism array P1by δ=5.7°. For small angles, the following relation applies:

δ=(n ₁ /n ₂−1)*α

where α is the prism angle and n₁ and n₂ are the refractive indices ofthe prism P1 and of the surrounding material, i.e. the glass G. Typicalvalues are n₁=1.65 and n₂=1.5, i.e. there is a refractive indexdifference of Δn=0.15. This results in a required prism angle of α=57°.

The company Ocuity has already produced birefringent lenticulars of asize of several inches for an application that is different from the onedescribed here. A sandwich of commercially available prism array, spacerglass plate and birefringent prism array can thus me made in a largesize in order to achieve beam combination.

The light wave multiplexing means can thus comprise a lens means and aprism means (see FIG. 18). The light 106 which passes though a firstmodulator cell can be focused by the lens means in a first region in aplane that lies downstream of the lens means in the direction of lightpropagation. The light 108 which passes through a second modulator cellcan be focused by the lens means in a second region in that plane. Theprism means is arranged at the plane. The prism means is designed suchthat the light of the first region is deflected by the prism means intoa first presettable direction, and the light of the second region isdeflected into a second presettable direction. The first and the secondpresettable direction are substantially identical. The first region isarranged at a distance to the second region. The lens means comprises alenticular L, and the prism means comprises a prism array P.

The light wave multiplexing means according to FIG. 19 comprises a firstprism means and a second prism means. The light 106 which has passed afirst modulator cell can be deflected by the first prism means into afirst direction. The light 108 which has passed a second modulator cellis not deflected. The first prism means is followed in the direction oflight propagation by the second prism means at a defined distance d. Thesecond prism means is designed such that the light 110 which has beendeflected by the first prism means can be deflected by the second prismmeans into a presettable direction. The light 112 which has not beendeflected is not deflected by the second prism means.

The second prism means comprises a prism array P2 with birefringentprism elements. The light 106 which passes through the first modulatorcell is polarised such that it can be deflected by a birefringent prismelement of the second prism means. The light 108 which passes throughthe second modulator cell is polarised such that it is not deflected bythe second prism means.

The first prism means comprises a prism array P1 with prism elements.The prism elements are arranged such that only the light 106 whichpasses though the first modulator cell is assigned to a prism element,and that the light 108 which passes though the second modulator cell isnot assigned to a prism element.

FIG. 20 shows another embodiment of the present invention. Here, thelight wave multiplexing means comprises at least two birefringent mediaSV1, SV2. One birefringent medium SV1 is arranged upstream of themodulator cells 1, 2, seen in the direction of light propagation, andanother birefringent medium SV2 is arranged downstream of the modulatorcells 1, 2. The birefringent media SV1, SV2 each have a presettableoptical property. The optical property of the birefringent medium SV1,which is arranged upstream of the modulator cells 1, 2, is chosen suchthat a first portion of the light is deflected by a first defined angletowards the first modulator cell 1. In an upper section of FIG. 20, thebeam diameter of this light portion is indicated by dotted lines. Twofurther beams are drawn in below, and all beams shall be construed to belike this across the entire surface of the element. Another portion ofthe light is not deflected. The beams of these light portions areindicated by full lines. The optical property of the birefringent mediumSV2, which is arranged downstream of the modulator cells 1, 2, is chosensuch that the other portion of the light is deflected by a seconddefined angle and that the first portion is not deflected. The opticalproperty of the two birefringent media SV1, SV2 shall in particular beunderstood to be the orientation of the optical axis or major axis ofthe respective birefringent medium SV1, SV2. The optical axes of the twobirefringent media SV1, SV2 are indicated by double arrows and havesubstantially the same orientation. There are other thinkableconfigurations, where the orientation of the optical axes of the twobirefringent media SV1, SV2 do not lie in the drawing plane of FIG. 20.Although it is generally possible that the light which runs towards thefirst birefringent medium SV1 is not polarised, it is preferablyprovided that the light which falls on the first birefringent medium SV1has a defined linear polarisation.

The two birefringent media SV1, SV2 in FIG. 20, but also thebirefringent media SP, SP1, SP2 and SP3 in FIGS. 13, 14, 17 and 21, havesubstantially coplanar interfaces.

Referring to FIG. 20, a retardation plate in the form of a λ/2 plate isarranged between the two birefringent media SV1, SV2. This retardationplate turns the direction of polarisation of the light which passesthrough the modulator cells 1, 2 by 90 degrees.

An aperture mask BA is arranged upstream of the first birefringentmedium SV1, seen in the direction of light propagation, said aperturemask being designed such that the non-deflected portion of the light,which would propagate towards each modulator cell 1, is blocked out. Inother words, the aperture mask BA comprises individual apertures whichhave substantially the same cross-sectional area as the modulator cells1, 2. Now, the aperture mask is positioned such that every othermodulator cell, i.e. all modulator cells 1 are covered so that no lightfalls on them. This is to prevent that non-deflected light passesthrough the modulator cells 1. The individual components are shownseparately in FIG. 20 in order to keep the drawing comprehensible.However, the components can be combined in the form of a sandwich, i.e.be in direct contact with each other.

In the arrangement shown in FIG. 20, the distance between the modulatorcells 1, 2 and a further optical element which is arranged downstream ofthe birefringent medium SV2 (e.g. a deflection prism cell arrangement oran apodisation filter, not shown in FIG. 20) can preferably be reducedcompared with an arrangement for example as shown in FIG. 17. Thearrangement shown in FIG. 20 is particularly preferable for beamcombination of spectrally broad-band light, but can also be used forspectrally narrow-band light. An arrangement shown in FIG. 20 serves torealise a symmetrical beam splitting and beam combination, which canserve to minimising the deviation of the optical path lengths on the onehand and/or of the superposed, i.e. combined wave fronts on the other.It can thus be achieved that the diffraction patterns of the twosuperposed modulator cells 1, 2 comprise the same intensity and phasedistributions (except the orthogonality of the polarisation state) atthe point of exit of the light modulator device. This is a major aspectfor a high-quality hologram reconstruction, if such a light modulatordevice is used in a holographic display. Similarly, minimisingcross-talking of light which passes through two adjacent modulator cells1, 2 of the arrangement is another important aspect of high-qualityhologram reconstruction.

FIG. 22 shows another embodiment which serves to realise a similarfunction as the embodiment shown in FIG. 20. The embodiment shown inFIG. 20 uses refractive components, namely the two birefringent mediaSV1 and SV2. In contrast, the embodiment shown in FIG. 22 usesdiffractive components, namely the deflection layers Vg1, Vg2, Vg3 andVg4 shown in the drawing, which are realised in the form of volumegratings. The deflection layers Vg1, Vg2 are arranged upstream of themodulator cells 1, 2, seen in the direction of light propagation. Thedeflection layers Vg3, Vg4 are arranged downstream of the modulatorcells 1, 2, seen in the direction of light propagation. The light whichfalls on the first deflection layer Vg1, i.e. which is not blocked bythe cover B, is not polarised but shows a homogeneous distribution ofindividual polarisation portions, or is has a defined polarisationstate, e.g. a linear polarisation.

The first deflection layer Vg1 is designed such that the light is splitup into two partial beams. The one partial beam is substantially notdeflected and it is linearly polarised, i.e. has for example a TEpolarisation; it is indicated by dotted lines in the drawing. The otherpartial beam is deflected by a defined angle and it is also linearlypolarised, but has for example a TM polarisation; it is indicated bybroken lines in the drawing. The second deflection layer Vg2 is arrangedparallel to the first deflection layer Vg1, and it is designed such thatthe light which has not been deflected is not deflected and the lightwhich has been deflected by the defined angle is deflected by anotherangle. The absolute values of the two deflection angles aresubstantially identical, namely 60°. The direction of polarisation ofthe light which has not been deflected is turned by 90 degrees by thestructured retardation plate, which is realised in the form of a λ/2plate and which is arranged downstream of the second deflection layerVg2. Consequently, the light which passes through the modulator cells 1,2 has a substantially identical polarisation state.

The modulator cells 1, 2 are designed such that they can modify thephase of the light which interacts with them. A further structuredretardation plate in the form of a λ/2 plate is arranged between themodulator cells 1, 2 and the third deflection layer Vg3, said plateturning the direction of polarisation of the light which passes throughthe modulator cell 2 by 90 degrees. The light falls on the thirddeflection layer Vg3, which is designed such that the light which passesthrough the modulator cell 2 is substantially not deflected, and thelight which passes through the modulator cell 1 is deflected by adefined angle. The fourth deflection layer Vg4 is arranged parallel tothe third deflection layer Vg3, and it is designed such that the lightwhich has not been deflected by the third deflection layer Vg3 is notdeflected and the light which has been deflected by the third deflectionlayer Vg3 by the defined angle is deflected by another angle. Theabsolute values of the two further deflection angles are substantiallyidentical. In this respect, the light beams which pass though the twomodulator cells 1, 2 are thus combined and propagate substantially inthe same direction. If the two modulator cells 1, 2 realisesubstantially the same phase value, the optical path lengths of the twopartial beams are substantially identical.

There are modulator cells 1, 2, or SLMs, which do not require a definedentry polarisation. In that case, it is possible to omit the structuredretardation plate upstream of the modulator cell plane and to replacethe second structured retardation plate, which is arranged immediatelydownstream of the modulator cell plane, by an unstructured retardationplate, i.e. an unstructured half-wavelength plate.

For an RGB presentation—i.e. when using light of differentwavelengths—it is possible to expose three different volume gratings,each being adapted to an individual wavelength, in an interleaved mannerin each of the deflection layers Vg1-Vg4. The arrangement shown in FIG.22 is of course also thinkable complemented in the form of columns,lines or matrices, namely when the components which are shown in FIG. 22continue above and below and/or out of the drawing plane—very much likein FIG. 20.

The light wave multiplexing means are typically arranged immediatelydownstream of the modulator cells of the modulation array, seen in thedirection of light propagation, in the drawings. However, it is alsopossible to arranged the light wave multiplexing means shown in thedrawings at a different position. For example, another optical componentcan be arranged between the modulation array and the light wavemultiplexing means. A light wave multiplexing means as shown in thedrawings and as claimed in the claims, can thus be arranged downstreamof that further optical component, seen in the direction of lightpropagation. Such a further optical component can for example be anillumination unit as disclosed in documents DE 10 2009 028 984.4 orPCT/EP2010/058619. The light which is injected into this illuminationunit can for example leave at right angles to its surface (which isarranged parallel to the modulation array) and propagate onto areflection-type modulation array. Once the light which comes from theillumination unit has been modulated by the modulator cells of themodulation array and reflected—for example by a reflective layer of themodulation array, the modulated light passes through the illuminationunit substantially without being deflected and then falls on the lightwave multiplexing means. In this case, the light wave multiplexing meansis arranged on the side of the illumination unit which faces away fromthe modulation array. To enable the light which is modulated andreflected by the modulation array to pass through the illumination unitwithout any obstructions, a film is provided between the illuminationunit and modulation array which turns the direction of polarisation forexample by 45° whenever the light passes through it.

Finally, it must be said that the embodiments described above shallsolely be understood to illustrate the claimed teaching, but that theclaimed teaching is not limited to these embodiments.

CITATIONS

-   [1] Chulwoo Oh and Michael J. Escuti: Achromatic polarization    gratings as highly efficient thin-film polarizing beamsplitters for    broadband light, Proc. SPIE, vol. 6682, no. 628211, 2007-   [2] Jihwan Kim et al.: Wide-angle nonmechanical beam steering using    thin liquid crystal polarization gratings, Proc. SPIE, vol. 7093,    no. 709302, 2008

1. A complex spatial light modulator comprising: a polarization phasemodulator which modifies incident light and separates the incident lightinto a first beam having a first polarization and a first phase, and asecond beam having a second polarization and a second phase, and whichoutputs the first beam and the second beam; and a beam synthesizercomprising a prism structure formed of an optical anisotropic material,the optical anisotropic material having a first refractive index withrespect to the first beam having the first polarization and a secondrefractive index, different from the first refractive index, withrespect to the second beam having the second polarization, where thebeam synthesizer combines the first beam and the second beam and outputsa beam.
 2. The complex spatial light modulator according to claim 1,further comprising a first polarizer, disposed on an optical path beforethe polarization phase modulator, which transforms a polarization of abeam incident thereon into a beam having the first polarization.
 3. Thecomplex spatial light modulator according to claim 1, wherein thepolarization phase modulator comprises: a phase-type spatial lightmodulator comprising a first pixel that modulates a phase of a portionof the incident light and outputs a beam having the first phase and asecond pixel that modulates a phase of a portion of the incident lightand outputs a beam having the second phase.
 4. The complex spatial lightmodulator according to claim 1, wherein the beam synthesizer comprises:a first prism array comprising at least one prism element that has alight-incident surface which is normal to an optical axis of theincident light, and a first inclined surface which is inclined withrespect to the light-incident surface; and a second prism array that isspaced apart from the first prism array, and that comprises at least oneprism element having a light-exit surface which is parallel with thelight-incident surface, and a second inclined surface facing away fromthe first inclined surface and which is inclined with respect to thelight-incident surface.
 5. The complex spatial light modulator accordingto claim 4, wherein the first prism array is formed of an isotropicmaterial and the second prism array is formed of an optical anisotropicmaterial having a first refractive index, with respect to the first beamhaving the first polarization, that is substantially the same as arefractive index of the surrounding material, and having a secondrefractive index, with respect to the second beam having the secondpolarization, that is chosen to be different to the first refractiveindex.
 6. The complex spatial light modulator according to claim 4,wherein an angle by which the first inclined surface and the secondinclined surface are inclined with respect to the light-incident surfaceis such that an optical path of the first beam is changed due torefraction at the first inclined surface and refraction at the secondinclined surface, and thus the optical path of the first beam ismodified, by the beam synthesizer, to be the same as an optical path ofthe second beam which passes through the first inclined surface and thesecond inclined surface without refraction.
 7. The complex spatial lightmodulator according to claim 1, wherein the beam synthesizer comprises:a first prism array comprising at least one prism element that has alight-incident surface which is normal to an optical axis of theincident light, and a first inclined surface which is inclined withrespect to the light-incident surface; a second prism array that isspaced apart from the first prism array, wherein the second prism arraycomprises at least one prism element having a second inclined surfacefacing away from the first inclined surface and which is inclined withrespect to the light-incident surface, and a light-exit surface which isparallel to the light-incident surface; and a structure which fills aregion between the first prism array and the second prism array.
 8. Thecomplex spatial light modulator according to claim 7, wherein thestructure is formed of an isotropic material, and where the first prismarray is formed of an isotropic material and the second prism array isformed of an optical anisotropic material with a first refractive index,with respect to the first beam having the first polarization, and with asecond refractive index, with respect to the second beam having thesecond polarization, that is different from the first refractive index.9. The complex spatial light modulator according to claim 8, wherein thebeam combiner comprises deflection layers, where a first lightdeflection layer changes the path of the first modulation light into adirection of the path of the second modulation light, where a seconddeflection layer changes the path of the first modulation light so thatthe first modulation light is outputted of the beam combiner parallel tothe second modulation light.
 10. The complex spatial light modulatoraccording to claim 9, wherein the an angle by which the first inclinedsurface and the second inclined surface are inclined with respect to thelight-incident surface is such that an optical path of the first beam ischanged due to refraction at the first inclined surface and refractionat the second inclined surface, and thus the optical path of the firstbeam is modified, by the beam synthesizer, to be the same as with anoptical path of the second beam which passes through the first inclinedsurface and the second inclined surface without refraction.
 11. Thecomplex spatial light modulator according to claim 1, wherein the beamsynthesizer comprises: a first prism that has a light-incident surfacewhich is normal to an optical axis of the incident beam, and a firstinclined surface which is inclined with respect to the light-incidentsurface; a second prism that has a second inclined surface which isinclined with respect to the light-incident surface, and a light-exitsurface which is parallel to the light incident surface, which is spacedapart from the first prism along a direction of the optical axis, andwhich is offset from the first prism; and a structure which fills aregion between the first prism and the second prism.
 12. The complexspatial light modulator according to claim 11, wherein the structure isformed of an isotropic material; and the first prism array is formed ofan isotropic material and the second prism array is formed of an opticalanisotropic material with a first refractive index, with respect to thefirst beam having the first polarization, and with a second refractiveindex, with respect to the second beam having the second polarization,that is different from the first refractive index.
 13. The complexspatial light modulator according to claim 11, wherein an angle by whichthe first inclined surface and the second inclined surface are inclinedwith respect to the light-incident surface is such that an optical pathof the first beam is changed due to refraction at the first inclinedsurface and refraction at the second inclined surface, and thus theoptical path of the first beam is modified, by the beam synthesizer, tobe the same as with an optical path of the second beam which passesthrough the first inclined surface and the second inclined surfacewithout refraction.
 14. A holographic three-dimensional image displaydevice comprising, a light source unit; the complex spatial lightmodulator of claim 1; and a controller which controls the complexspatial light modulator, to modulate a light beam from the light sourceunit according to three-dimensional information.
 15. The holographicthree-dimensional image display device according to claim 14, whereinthe light source unit outputs a beam that is polarized in the firstpolarization direction.